This invention relates to electron beam sources and, more particularly, to photocathodes for the generation of electron beams.
Electron beam (e-beam) sources are used in several fields, including scanning electron microscopes, defect detection instruments, VLSI testing equipment and electron beam (e-beam) lithography. In general, e-beam systems include an electron beam source and electron optics. The electrons are accelerated from the source and focused to define an image at a target. These systems typically use a physically small electron source having a high brightness.
Improvements in optical lithography techniques in recent years have enabled a considerable decrease in the line widths of circuit elements in integrated circuits. Optical methods will soon reach their resolution limits. Production of integrated circuit elements with smaller line widths (i.e., those with line widths less than about 0.1 xcexcm) will require new techniques such as X-ray or e-beam lithography, which can provide accompanying resolutions well below 1 xcexcm because of the shorter wavelengths associated with X-rays or electrons.
In e-beam lithography, (also called charged particle beam lithography), a beam of e.g. electrons from an electron source is directed onto a substrate. The electrons expose a resist layer (in this case an electron sensitive resist) on the substrate surface. Electron beam lithography uses what are called xe2x80x9celectron lensesxe2x80x9d to focus the electron beam. These are not optical (light) lenses but are either electro-static or magnetic. Typically, electron beam lithography is used for making masks; however it can also be used for direct exposure of semiconductor wafers.
Since modem lithographic systems must achieve fast writing times (high throughput rates) in addition to high resolution, their electron beams must also have a high brightness, which in the case of electron beams requires a high current density. This property is especially important for so-called direct write applications in which the electron beam is rapidly scanned and modulated so as to effect a projection of the image of a highly complex circuit directly onto a semiconducting chip substrate.
The primary motivation for using multiple beams in an electron-beam lithography system is to increase the total current that can be delivered while minimizing space-charge effects in each beam. FIG. 1 depicts in a side view one type of so-called xe2x80x9chybridxe2x80x9d multiple e-beam lithography, in which multiple electron beams are created by focusing an array of light beams, where each light beam""s intensity can be independently regulated, onto a photocathode in transmission mode (wherein the photocathode is back-illuminated with the light beams which are focused on a photoemission layer). The electron beams emitted from the photoemission layer are then accelerated, focused, and scanned across the wafer or mask using a conventional electron-optical column.
A conventional mask 118 (reticle) of the type now used in photolithography is positioned on a conventional stage 124 which may or may not be movable along one or both of the depicted x and y axes, depending on the type of photolithography subsystem. A source of the light is, for instance, a conventional UV light source or a laser illumination system 114 of the type now used in photolithography, which provides a relatively large diameter beam 116 of, for instance, laser illumination light which passes through the transparent portions of the mask 118. It is to be understood that the mask is a substrate transparent to the incident light 116 on which are located opaque areas. The transparent portions of the substrate define the image which is to be transferred by the mask 118. Typically, one such mask includes the entire pattern of one layer of a single integrated circuit die. The mask is usually, in terms of its X, Y dimensions, some convenient multiple of the size of the actual die being imaged.
A light optical lens system 128 (which is actually a lens system including a large number of individual lens components) focuses the light 126 passed by the mask 118. The light optical lens system 128 is either a 1:1 or demagnifying lens system which demagnifies by e.g. a factor of four or five the image 126 incident thereon to form image 130, which in turn is incident onto the object. A 1:1 ratio is more advantageous when mask size is limited. In this case the object, rather than being a semiconductor substrate, is the photosensitive backside of a photoemission cathode 132. The photoemission cathode 132 defines for instance a minimum feature size of 0.5 micrometers or less, the minimum feature size of course being dependent upon the parameters of the system. The photoemission cathode 132 is for example a thin gold (or other metal) layer deposited on a transparent substrate.
The photoemission cathode 132 (which like the other elements herein is shown in simplified fashion) includes a photoemission cathode layer 134 which absorbs the incident photons 126 and causes electrons present in the photoemission layer 134 to be excited above the vacuum level. Some portion of the electrons 138 which retain sufficient energy to escape from the photoemission layer 134 are emitted into the vacuum portion 140 of the photoemission cathode downstream from the photoemission layer 134. An electric voltage (typically several kilovolts to tens of kilovolts) is applied to the extraction electrode 142 associated with the photoemission cathode 132. Extraction electrode 142 extracts the electrons 138 which have escaped from the photoemission layer 134 and accelerates them. Thus the accelerated electrons 146 form a virtual image of the incident photons 130. In effect then the photoemission cathode 132 and extraction electrode 142 form a divergent lens.
There may also be, immediately downstream of the extraction electrode 142, a magnetic (or electrostatic) lens (not shown) to reduce aberrations. (A magnetic lens is conventionally a set of coils and magnetic pole pieces, and yokes which focus the electron beam.) Such an electron beam system has been found to offer resolution of below 10 nm. Immediately following (downstream of) this portion of the system is a conventional electron optical lens system 150 consisting of one or more electron lenses and alignment, deflection and blanking systems 152 (shown only schematically in FIG. 1).
This lens system further demagnifies the virtual image 146 at the writing plane, which is the plane of the principal surface of the wafer 158 by a factor determined to achieve the desired minimum feature size. For instance, if a minimum feature size of 0.5 xcexcm is resolved at the photoemission cathode, an electron beam demagnification factor of five times is needed for a 100 nanometer minimum feature size on the wafer 158. This means that when a total area of approximately 1 mmxc3x971 mm is exposed on the wafer 158, a total illuminated area of 5 mmxc3x975 mm is required on the photoemission cathode layer 134. Correspondingly for a 4:1 demagnification ratio an area of 20 mmxc3x9720 mm is illuminated on the mask 18, and a 5 mmxc3x975 mm area is illuminated for a 1:1 ratio. Of course these are merely illustrative parameters.
The total demagnification factor and exposed wafer area can be varied to achieve the desired minimal feature size and throughput. The wafer 158, including its electron beam resist layer 160, is typically supported on a stage 164 which is movable in the x, y and z axes, as is conventional. Other elements of both the photo and the electron beam subsystems which are well known are not shown, but include positioning measurement systems using for instance laser interferometer to determine the exact location of the mask on its stage and the wafer on its stage, vacuum systems, air bearing supports for the stages, various vibration absorption and isolation mechanisms to reduce environmental effects, and suitable control systems, all of the type well known in the lithography field.
The deflection system 152 can be used to compensate for positionary errors due to mask/wafer misalignment, vibrations, heating and other effects, and would only use very small deflection amplitudes.
FIG. 2 shows in a side cross-sectional view a portion of a photocathode 200 having an optically transmissive substrate 201 and a photoemission layer 202. The photocathode array 200 is back-illuminated by light (laser) beams 203 (having an envelope defined as shown) focused on photoemision layer 202 at irradiation region 205. As a result of the back-illumination onto photoemission layer 202, electron beams 204 are generated at an emission region 208 opposite each irradiation region 205. Other systems are known where the photoemitter is front-illuminated, i.e., the light beams are incident on the surface of the photoemitter from which the electron beam is emitted.
Photoemission layer 202 is made from any material that emits electrons when irradiated with light. These materials include metallic films (gold, aluminum, etc.) and, in the case of negative affinity (NEA) photocathodes, semiconductor materials (especially compounds of Group III and Group V elements such as gallium arsenide). Photoemission layers in negative electron affinity photocathodes are discussed in Baum (U.S. Pat. No. 5,684,360).
When irradiated with photons having energy greater than the work function of the material, photoemission layer 202 emits electrons. The resulting electron beam is shown below region 208 and has a lateral extent shown by the lines crossing at region 208. Typically, photoemission layer 202 is grounded so that electrons are replenished. Photoemission layer 202 may also be shaped at emission region 208 in order to provide better irradiation control of the beam of electrons emitted from emission region 208.
Photons in light beam 203 have an energy of at least the work function of photoemission layer 202. The number of emitted electrons is directly proportional to the intensity of the light beam. Photoemission layer 202 is thin enough and the energy of the photons in light beam 203 is great enough that a significant number of electrons generated at irradiation region 203 migrate and are ultimately emitted from emission layer 208.
Transparent substrate 201 is transparent to the light beam and supports the photocathode device within an electron beam column which may be a conventional column or a microcolumn. Transparent substrate 201 may also be shaped at its surface where light beams 203 are incident in order to provide focusing lenses for light beams 203. Typically, transparent substrate 201 is a glass although other substrate materials such as sapphire or fused silica are also used.
One of the critical challenges in developing a photocathode as the electron beam source in multiple electron beam lithography featuring high current is the ability to conduct heat away from the focused regions of illumination on the photocathode. The laser power needed to produce a certain beam current depends on the conversion efficiency of the photocathode material. A considerable amount of energy per unit area is dissipated in these regions due to the relatively low conversion efficiency of the photoemission process. For example, if a gold film approximately 15 nm in thickness is used as the photoemission layer 202, the efficiency is about 5xc3x9710xe2x88x925, which implies that 5 mW of laser beam power is needed to produce a 100 nA electron beam.
When this amount of power is focused into a small spot (approximately 1 xcexcm diameter) on a thin film, the heat flow is limited by conduction through the cathode support material. This conduction path is inefficient due to the generally low thermal conductivity of common optically transparent substrate materials such as fused silica (glass). Consequently, a significant temperature rise will occur at the photocathode. For a fused silica substrate and a 1 xcexcm spot size, a 15 nm gold film (used as the photoemitter) will be heated on the order of 1000xc2x0 C. Without adequate cooling, the resulting temperature rise could degrade or even destroy the photocathode. This may impose a severe limit on the total current that can be generated in each beam and thus may limit the overall throughput of the lithography system. Clearly, there is a need for adequate cooling for the photocathode.
While others have contemplated the use of thermoelectric devices for cooling photocathodes (wherein the thermoelectric device is directly secured to the photocathode, or a layer of electrically insulating but thermally conductive material is interposed between the thermoelectric device and the photocathode-see Ace, U.S. Pat. No. 3,757,151), the physical size of these devices prevent their use in e-beam lithography.
According to the present invention, a photocathode device is operable to emit multiple high current density beams of electrons upon illumination with appropriate light, and there is an associated electron beam generator which includes the photoemissive cathode and is suitable for a semiconductor lithography system. The photocathode device includes an optically transmissive substrate material which is patterned to have protrusions. Spaces between the protrusions are at least partly filled with material that has a high thermal conductivity (copper, gold, or platinum, for example). The photoemitter is deposited over the protrusions.
The photoemitter is positioned on the surface of the substrate opposite the surface receiving illumination, and thereby has an irradiation region at the contact area with the optically transmissive substrate, and an emission region opposite the irradiation region, these regions being defined by the axis of the light beams. Heat is generated by the light beams incident at this interface. The diameter of the tip of the protrusion in one embodiment defines the areal extent of the emission region.
An advantage of this structure is that spaces between the protrusions are occupied by material having a high thermal conductivity (copper, gold, or platinum for example). If the thickness of the heat conducting material is large in relation to the diameter of the protrusion on the substrate, then heat conduction away from the irradiation region/emission region interface is dominated by the lateral flow of heat in the conducting material. The thermal conductivities of copper, gold, and platinum are at least 200 times greater than that of fused silica, which is an alternative substrate material for photocathodes. Accordingly, a proportionally lower temperature rise at the irradiation region of the photoemissive layer is expected for a given incident power and laser spot size. This efficiently conducts heat away from the irradiation region/emission region interface, and therefore allows higher operating currents to be achieved from the photocathode. This, in turn, permits higher throughputs in applications including electron beam lithography.
The present structure advantageously can be fabricated using standard microfabrication techniques. For example, the substrate protrusions can be patterned by conventional microlithography followed by reactive ion etching. Heat conducting material can be deposited by, for example, sputtering or thermal evaporation. Chemical mechanical polishing can be used to remove heat conducting material from the tip of the protrusions. Alternatively, the heat conducting material could be deposited by electroplating. The final step is depositing the photoemitter. By using conventional microfabrication techniques, the emission spot size (which coincides with the tip of the protrusions on the substrate) can be reduced to less than 1 xcexcm diameter. With this small emission spot size, a lower column demagnification is needed to achieve a given beam size at the target wafer. Lower column demagnification can be achieved in a shorter column, which advantageously decreases total blur due to electron-electron interactions. Accordingly, a higher throughput can be realized as a greater number of emission points is used, or a larger area on a target wafer can be written at one time.
The invention and its various embodiments are further discussed along with the following figures and accompanying text.